Short cage RD to Long, chain length?

I am switching from a 7 speed_(13 to 24) freewheel to a new 9 speed_(12 to 23) cassette. A new wheel as well. I have an Ultegra long cage RD that was next to free so I will use it even though it's a standard double up front.

Does the new 9 speed chain need to be any longer than the previous 7 speed chain or can I just use the old one as a guide for the length.

Also I am going from 42_53 double to 39_53 chainrings. I doubt if that factors into anything.
Thanks.

Not wearing a helmet makes me more aware of my surroundings. I find myself anticipating the hardness of concrete 50 or 100 feet in front of me, it's almost a zen-like connection between my face and the pavement.

The small/small combo is the better way to set the chain length. In most cases, you want the longest length that will move the RD cage by a small amount when the ends are brought together, but the chain must not hang loose or rub the upper chain guide tab. This produces the maximum wrap capacity from any RD. Using the big/big plus one inch method will only suggest that length with the largest cog within the RD's wrap capacity.

I use the big/big + 1 method because it produces the shortest possible chain.

Originally Posted by slopvehicle

Not wearing a helmet makes me more aware of my surroundings. I find myself anticipating the hardness of concrete 50 or 100 feet in front of me, it's almost a zen-like connection between my face and the pavement.

Not if just testing it. But running in the big~big combo is a no~no anywho. As long as the RD isn't impacting with the cassette or similar phenomena, all should be well.

Horrible logic. Here's why:

It's a beautiful sunny spring day. The flowers are blooming and the birds are singing. There's no wind and you are enjoying your best ride ever. Cruising along in your big ring you decide to sprint up a small hill. As you approach the top of the hill your cadence drops off so you shift down. You feel a little resistance from the pedals so you push just a bit harder to get you over the top. SURPRISE! You have just inadvertantly shifted into your big/big combination. The chain isn't long enough and the easiest thing to bend is the derailleur arm. It goes into the spokes destroying your wheel and damaging your frame. As you start the long walk home a cloud passes over the sun and a cold rain starts to fall.